UESL is a relatively new clinicopathologic entity that was first described in 1978 by Stocker and Ishak . It is found most frequently in children between 6 and 10 years of age  but has been rarely reported in adults. There have been 46 articles regarding adult UESL published from 1977 to 2015 . UESL accounts for fewer than 1% of all primary liver neoplasms in adults . In a recent review on UESL, the median age of the study group was 25, with the oldest patient being 86 years old . In most cases, symptoms related to the tumor are nonspecific, with abdominal pain and abdominal mass reported to be the most common presenting complaints [2, 7, 8]. There are no specific tumor markers. Furthermore, the results of imaging studies such as CT, US, and MRI are often inconclusive . On US, UESL usually appears as a hypoechoic solid mass . CT characteristics include the presence of an enhanced peripheral rim, some solid portions at the periphery or adjacent to the septa, and a predominantly unilocular or multilocular appearance [9, 10]. These features occasionally lead to this tumor being misdiagnosed as a benign cystic lesion. In this case, because of its multilocular cystic appearance on CT imaging, mucinous cystadenocarcinoma of the liver should be considered in the differential diagnosis. In addition, since UESL in adulthood is very rare, it was extremely difficult in this case to make a preoperative diagnosis. However, the intermixed appearance of hemorrhage and mucinous content on MRI and the honeycomb appearance with solid components on US studies were the characteristics of UESL. The discrepancy between the solid appearance on US and cystic appearance on CT in particular should raise suspicion for this tumor .
The prognosis of UESL is very poor, with a median reported survival time of less than 1 year . Some case reports and studies demonstrated long-term survival of patients who underwent only surgery . However, even if a complete resection is achieved in surgery, UESL recurs in many cases . Lenze et al. reviewed reports from 1955 to 2007 and reported that the combination of surgical resection and adjuvant chemotherapy may improve prognosis compared with radical tumor resection alone . Although there were no standard regimens for adjuvant chemotherapy, a sarcoma-directed chemotherapy such as combination of vincristine, actinomycin, ifosfamide, and doxorubicin or combination of carboplatin and etoposide was used [1, 6]. In addition, multidisciplinary therapy such as chemotherapy, radiotherapy, interventional therapy, and transplantation may be helpful for improving survival for unresectable recurrent tumors . However, adjuvant chemotherapy was not administered to our patient because there were no standard regimens for UESL. Repeat complete resection for recurrent tumor was performed, and now, the patient has no recurrent tumor after 26 months following the first operation.
The pathological features of UESL are poorly understood. In the case reported here, mesenchymal hamartoma-like lesions were partially observed. Several authors have suggested an evolutive histological spectrum between MHL and UESL, based on histological findings [13, 14]. However, the occurrence of MHL in adults is also very rare. Furthermore, there are some reports of a benign postoperative course of MHL after incomplete resection [15–19]. In the liver metastatic lesion of this case, there were some mesenchymal hamartoma-like lesions. A UESL-like lesion and a mesenchymal hamartoma-like lesion coexisted not only in the primary tumor but also in the metastatic tumor, suggesting that a mixed mesenchymal hamartoma-like lesion is one form of UESL, rather than a malignant transformation of MHL. Further investigation is needed to clarify the pathogenesis and relationship of MHL and UESL.